Improved documentation on native dataset support

[schlunma]

Description

The documentation on how to include native datasets is sparse and also wrong at some points. This PR fixes this.

Link to doc: https://esmvaltool--2635.org.readthedocs.build/en/2635/input.html#datasets-in-native-format

Requires ESMValGroup/ESMValCore#1559.

Checklist

It is the responsibility of the author to make sure the pull request is ready to review. The icons indicate whether the item will be subject to the 🛠 Technical or 🧪 Scientific review.

• 🛠 This pull request has a descriptive title
• 🛠 Code is written according to the code quality guidelines
• 🛠 Documentation is available
• 🛠 Tests run successfully
• 🛠 The list of authors is up to date
• 🛠 Any changed dependencies have been added or removed correctly
• 🛠 All checks below this pull request were successful

---

To help with the number of pull requests:

• 🙏 We kindly ask you to review two other open pull requests in this repository

[bouweandela]

Thanks!